1. Field of the Invention
The present invention relates to the design of an antenna driver used in a Reader Tx block of an NFC (near field communication) device. The antenna driver, which is part of the Reader Tx block, drives an antenna to output a magnetic field to establish communication with a Tag Tx block included in another NFC device.
2. Background Art
FIG. 1A shows a conventional communication system 100 including two NFC devices 110, 120 for communicating with each other. Each of the NFC devices 110, 120 includes a Tag Tx block 112, 122, a Reader Tx block 114, 124, a Reader Rx block 116, 126, and a Tag Rx block 118, 128 along with other supporting circuitry (not shown). The communication between the NFC devices 110, 120 is initiated when an antenna driver included in a Reader Tx block drives an antenna of one of the NFC devices to output a magnetic field that can power a Tag Rx and Tx-block included in the other NFC device. The communication is established when the powered Tag Tx block modulates the magnetic field with a communications signal and transmits the modulated signal to the Reader Rx block. For example, to initiate communication, the Reader Tx1 block 114 includes an antenna driver that drives an antenna associated with the NFC device 110 to output a magnetic field that powers the Tag Tx2 block 122 and Tag Rx2 block 128 included in the NFC device 120. The communication is established when the Tag Tx2 and Rx2 are powered by the output magnetic field, and when Tag Tx2 block_122 load modulates the magnetic field with a communications signal which is read back by the Reader Rx1 block 116.
Alternatively, in the communication system 150 illustrated in FIG. 1B, the NFC device 110 may communicate with a Radio-Frequency Identification (RFID) device. The RFID device 130 a Tag 134, and other circuitry (not shown). The communication between the NFC device 110 and the RFID device 130 is similar to the communication between the NFC device 110 and the NFC device 120 discussed above. In particular, to initiate communication, the Reader Tx1 block 114 includes an antenna driver that drives an antenna associated with the NFC device 110 to output a magnetic field that powers the Tag block 134 included in the RFID device 130. The communication is established when the Tag block 134 is powered by the output magnetic field, and when Tag block 134 modulates the magnetic field with a communications signal and transmits the modulated signal back to Reader Rx1 block 114. The RFID device 130 can be similar to a RFID device according to ISO 14443, ISO 15693, or a contactless RFID smart card.
FIG. 2 shows a conventional topology of the Reader Tx block 114 included in the NFC device 110. The NFC device 110 also includes a battery 220 and an antenna driver 240, where the antenna driver 240 includes a power amplifier (PA) 230 powered by the low-dropout regulator (LDO) 210, the LDO 210 being powered by the battery 220. The power amplifier 230 of the antenna driver 240 drives the antenna 200 with an electrical signal to output the magnetic field, and initiate communication with the Tag Tx2 block 122 and Tag Rx2 block 128 included in the NFC device 120. In order to power the Tag Tx2 and Rx2 blocks 122 and 128, the antenna 200 is required to output a very strong magnetic field due to system inefficiencies and other factors such as the distance between the communicating NFC devices 110, 120. Therefore, the power amplifier 230 is required to drive the antenna 200 with a high voltage and current to output a high magnetic field. However, in the conventional topology, the driving capability of the power amplifier 230 is limited by the maximum LDO 210 output voltage (VLDOmax) which can be significantly lower than the maximum battery voltage (VBatmax). As such, to output a magnetic field at a given amount of power, the power amplifier 230 is required to output a higher amount of current due to the limitation in the driving voltage, which is inefficient given that a larger battery voltage of 5.5V is available.
FIG. 2 also shows a graph of output voltage for power amplifier 230 in the conventional topology. Because the power amplifier 230 is powered by the LDO 210 having a maximum voltage of 2.5V, the peak-to-peak output from the power amplifier 230 cannot exceed 2.5V. Further, in practice, the output voltage of the LDO 210 (VLDOreg) is regulated to be lower than the minimum battery voltage (during a normal discharge cycle) to prevent the LDO 210 from dropping out and clipping the output of the power amplifier 230. The minimum battery voltage may reach as low at 2.5 volts during discharge, and therefore VLDOreg may be set as low at 2.0V. As such, during the complete operation range of the battery voltage from 2.5V to 5.5V, the maximum peak-to-peak output from the conventional power amplifier 230 remains constant at the regulated voltage (VLDOreg) which is below the minimum battery voltage. In particular, the peak-to-peak voltage output by the power amplifier 230 remains constant at VLDOreg of about 2V and is not capable of tracking the changes in the battery voltage. Therefore, due to the limitation in output voltage, in the conventional topology, a higher amount of current is required to output the magnetic field at the given amount of power. The conventional topology does not allow for reduction in the overall current consumption of the circuit leading to the circuit being very inefficient.
Therefore, there is a need to improve the efficiency of the antenna driver design by reducing the overall current consumption, while providing a sufficient electrical signal to drive the antenna for magnetic field generation.